This invention relates to protective coatings for optical devices consisting of beryllium and other low Z (atomic number) elements for the purpose of enhancement in x-ray applications. More particularly, it pertains to protecting beryllium (or any other low Z element) with a coating having a) good transmission for long wavelength x rays, b) hardness to permit a higher resistance to abrasion, c) the effect of sealing the surface to give a higher resistance to corrosive elements and a surface free of pinholes and other anomalies which may limit its usefulness, and d) the ability to reduce or eliminate secondary emission from the transmissive element. This invention also includes several uses of the coating when applied to beryllium that make possible new and improved optical device x-ray tube and detector designs.
One application of beryllium and other low Z elements has been in connection with optical elements called window structures used for the transmission of radiation, such as x rays, from radiation generation elements. These optical elements are typically embodied in an optical device x-ray tube in which a vacuum environment along with a source of high energy particles, typically electrons, and a target which will generate the desired energy spectrum of x rays, is contained. Such windows typically need to satisfy the difficult requirements of being strong enough to withstand the environmental pressure differences between the inside and the outside of the tube and the chemical environment outside the tube, while being transmissive to the x rays generated by the target. Additionally, the window should not interfere with the prospective applications by creating secondary x rays within the range of energy which is of interest to the user nor should the window degrade over time with the continuously high rate of radiation which it receives.
In a similar application, the source of radiation is an electron microscope where high velocity electrons impinge on the specimen being examined and generate x rays characteristic of the atomic composition of the specimen. The optical device x-ray detector assembly part of an x-ray fluorescent spectrometer is typically placed as close as possible to the specimen. This detector assembly consists of an x-ray transparent port which separates the vacuum in the microscope from the vacuum in the detector assembly, an energy dispersive or wavelength dispersive detector element and the associated electronic connections and heat dissipation elements. This x-ray fluorescent spectrometer is used to analyze the emergent x rays and determine the composition of the specimen.
It is well-known that materials to be analyzed by the technique of x-ray fluorescence are more excited and thus more readily detected when the energy of the exciting radiation is close to the energy at which the material re-emits its characteristic x ray. The desire to detect elements with low atomic numbers has created the need to irradiate the sample with lower energy (longer wavelength) x rays. These are precisely the x rays that are most attenuated by any material. Therefore, a thinner window on an x-ray tube with the same applied rated power (KeV X ma) as presently exists would result in a significantly increased emission of the longer wavelength radiation (assuming an appropriate target which will generate such wavelengths). Such increased emissions would result in the better excitation of the light elements in the sample with a corresponding higher flux of the elemental characteristic radiation. This higher flux would have a greater probability of detection by the appropriate type of detector, resulting in an improved lower limit of detection for the light element sample in the spectrometer.
Thus, both the elements to be detected and the elements used in the transmissive structures are necessarily in the low Z area. Z refers to the atomic number of the element in the periodic chart. Low Z more specifically relates to those elements with an atomic number of less than 15 (hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminum, and silicon). Such elements should also be stable for the intended use and may be either single element structures or compound structures consisting of a preponderance of low Z material.
In other x-ray applications, an optical element such as a coated window, even of a beryllium thickness unacceptable for soft x-rays, would still be useful in the harsh environments sometimes found in process control, or where it is desired to water cool the window when the window itself is the x-ray generating anode target or, as in some high power, side window x-ray tubes, the power rating of the tube is limited by the ability of the window to dissipate heat caused by the bombardment of electrons reflected from the tube target.
The conventional technology for optical element x-ray tube windows used in high power, end window or low power, side window x-ray spectrometer tubes for light element applications, utilizes beryllium foil that has been rolled to a typical thickness of 5 mils (127 microns) and then brazed and/or compression bonded to a monel or stainless steel support structure which is or will become part of the tube envelope. This thickness is a compromise between a thicker dimension for providing a strong, sufficiently long lived structure for the high vacuum which exists inside the tube and a thinner dimension so as to permit a maximum amount of long wavelength x rays to escape from the tube. Although attempts have been made to manufacture tubes with thinner optical element windows, these have not been entirely successful because of either low manufacturing yield or unsatisfactory lifetimes for the in-use tube. The main short and long term failure mode for thin window x-ray tubes has been the development of a leak in the Be window resulting in sufficient air entering the tube to produce high voltage breakdown.
The manufacture of the Be foil used in the optical element x-ray tube windows has been accomplished by compressing beryllium powder of approximately 99.5% purity into billets and then rolling the billets until the desired foil thickness is achieved. After rolling, the beryllium grain sizes average 3.5 microns thick and 40 microns wide. The 0.5% contaminants (mostly oxygen) are generally segregated at the grain boundaries.
The vacuum leaks typically follow paths along the grain boundaries. The thinner the window, the shorter the paths and the higher the probability of leaks. Condensation of moisture on the external surface of the optical element x-ray tube window, particularly in the presence of salts whether from the environment or from human contact, contributes to the generation of leaks by electrolytic action between the beryllium grains and the metal window support as well as between the grains and the contaminants at the grain boundaries. The beryllium, which is chemically active, is readily dissolved by the galvanic action of the salt solution creating leaks in initially "vacuum tight" windows.
A prior art approach to resolving this problem has been the coating of the beryllium with another material that will protect the beryllium from exposure to the corrosive effects of moisture and yet will withstand the detrimental effects of high radiation dosage from the tube. Many types of coating have been tried without success, such as diamond and amorphous boron nitride.
A chemically active environment as is required to deposit diamond films can lead to the etching of the beryllium at the same time as the diamond is deposited. This makes the achievement of a thin, pinhole-free film problematic. Diamond is deposited within an environment rich in atomic hydrogen which, in addition to etching the beryllium, can produce volatile and potentially toxic beryllium hydrides. Boron nitride requires deposition at high tensile stress to avoid the creation of pinholes. With high stress, however, the boron nitride is subject to post deposition delamination.